Mars is the fourth planet from the Sun. It is
only about half as big as Earth and has only
about one-quarter of our planet's surface area. But because Mars doesn't
have any oceans or seas, the total area of the martian surface is about
equal to that of Earth's dry land.

Mars is named after the Roman God of War, probably because of its red color
which suggests blood and battle. It's also commonly known as the Red Planet.
Its ruddy color is due to large amounts of red iron oxides (rust), including
minerals such as hematite, in the surface
rocks and soil.

Mars has two tiny satellites, Phobos and Deimos, both believed to be captured asteroids.

Orbit

Orbits of the inner planets

Mars orbits about one and half times farther from the Sun than Earth does,
which is the main reason it's so much colder. The orbit of Mars is also
noticeably more elliptical than Earth's. This means the amount of solar
heat received during the day at any given point on the planet's surface
varies widely between when Mars is closest to the Sun (perihelion)
and when it's farthest away (aphelion).
Big swings in temperature are the result.

Although the yearly average temperature over the whole surface is about
-55°C (-67°F), the temperature ranges from as low as -133°C
(-207°F) at the winter pole to a surprisingly balmy 27°C (80°F)
at the equator on a summer day. These values are generally much lower than
would occur on Earth if our planet orbited the Sun at the distance of Mars,
due to the relatively feeble greenhouse
effect of the thin martian atmosphere.

One martian year equals just under two Earth years because Mars takes 687
days to complete an orbit.

When Mars is opposite the Sun in our sky it is said to be in opposition.
This is also when Mars and Earth are closest together and therefore a good
time for making observations. Before the age of space exploration, much
of our knowledge about Mars came from studies carried out at these favorable
times. For more details, see Mars, oppositions.

Rotation

Mars spins around on its axis once every 24.62 hours. This is the length
of the martian day, which is known as a sol. One sol is just 39 minutes
longer than an Earth day of 24 hours.

Topography

Mars as seen by the Hubble Space
Telescope

One of the most striking things about Mars is that its two halves are quite
different. Most of the southern hemisphere is marked by ancient cratered
highlands, similar to those on the Moon. The northern hemisphere, in contrast,
is dominated by lava-filled plains that reveal a more complex history of
activity and change. At the boundary between the two types of terrain is
an abrupt shift in of several kilometers. Scientists still aren't sure about
the origin of this sharp boundary and the two contrasting hemispheres. One
idea is that Mars suffered a massive impact shortly it formed.

From Earth, the northern plains of Mars look paler than the rest of the
planet and were once thought to be continents. Consequently, they were given
names such as Utopia Planitia (plain of Utopia) and Arabia Terra (land of
Arabia). The darker features in the south were believed to be seas and so
were given names such as Mare Erythraeum, Mare Sirenum, and Aurorae Sinus.
The largest dark feature on Mars seen from Earth, and the first permanent
marking on any planet to be spotted through a telescope, is Syrtis
Major.

Surface features

Mars has some outstanding surface features – quite literally. Among
its lofty volcanoes is the biggest in the
solar system, Olympus Mons. Mars also
boasts a collection of volcanoes in the northern Tharsis region so huge
that they make a noticeable bump in the planet's roundness; an impact crater
in the southern hemisphere, Hellas Planitia,
which is more than 6km deep and 2,000km wide; and a huge canyon system, Valles Marineris (Valley of the
Mariners), long enough to stretch from Los Angeles to New York. Mars may
also possess caves. For more about the
major surface features on Mars, see the table below and the accompanying
links.

Argyre Planitia

An 800km-diameter impact basin in the southern hemisphere
that is the youngest such structure on the planet, with an age of
about 3.5 billion years.

A relatively smooth, circular plain, possibly an
ancient impact basin, in the north equatorial region, 1,600km across
and 2.5km below the mean level of the planet's surface. It seems to
have suffered water erosion in the past and was the site of the Viking
1 landing. More here.

The second largest volcanic region on Mars, after Tharsis
Montes. It measures 1,700 by 2,400km and contains three large volcanoes:
Elysium Mons, Hecates Tholus (NE of Elysium Mons), and Albor Tholus
(SE of Elysium Mons). More here.

Formerly known simply as "Hellas," a near-circular
impact basin, some 2,500km wide and about 6km deep. It is conspicuous
by its color which often appears lighter than surrounding areas due
to overhanging mists and cloud. More here.

Ma'adim Vallis

One of the largest valley systems on Mars. Named after
the Hebrew for "Mars," it is about 860km long, 8 to 15km
wide, up to 2,100m deep, and located in the highlands of the southern
hemisphere. Images sent back by Mars Global Surveyor suggest that
Ma'adim Vallis formed some 3.5 billion years ago when a large lake,
estimated to have been 1.1 million km2 in area and 1,100m
deep, overflowed a low point in its perimeter.

The highest volcano in the solar system – three
times higher than Mount Everest. Mons Olympus wouldn't be difficult
to climb, though, because it rises very gently from the surrounding
plain. The only exception to this is a steep escarpment that borders the summit. A shield volcano, similar to those in the
Hawaiian chain but vastly larger, it measures 624km wide and 25km
high. In the center is a caldera, 80km wide with multiple circular,
overlapping collapse craters created by different volcanic events.
The radial features on the slopes of the volcano were formed by overflowing
lava and debris. Olympus Mons is found in the Tharsis Montes region
near the martian equator. More here.

A conspicuous dark, roughly triangular marking, about
1,200km long and 1,000km wide, centered at about +10° N, 290°
W; it was first noted by Christiaan Huygens in 1659. Formerly known
as the Hourglass Sea or the Kaiser Sea, its present name is Greek
for great sandbank, which is appropriate since it appears
to consist of a large area of wind-blown dust. More here.

An extensive upland region, from which rise three giant
shield volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons, each
to about 27km above the datum level for the planet. Tharsis Montes,
also known as the Tharsis Ridge, extends for 2,100km and varies in
height between about 9km and 11km above the datum level. More here.

The largest system of canyons in the solar system.
Just south of the martian equator, it is about 4,000km long –
as wide as the continental United States. The central individual troughs,
generally 50 to 100km wide, merge into a depression as much as 600km
wide. In places the canyon floor reaches a depth of 10km – six
to seven times deeper than the Grand Canyon. The geologic history
of the central canyon system is complex: first the surface collapsed
into a few deep depressions that later became filled with layered
material, perhaps as lake deposits. Then graben-forming faults cut
across some of the older troughs thus widening existing troughs, breaching
barriers between troughs, and forming additional ones. At that time
the interior deposits were locally bent and tilted, and perhaps water,
if still present, spilled out and flowed toward the outflow channels.
Huge landslides fell into the voids created by the new grabens. More here

Mars Pathfinder view of the surface of Mars. The
scene includes bouldery ridges and hummocks of flood debris that
range from a few tens of meters away to the "Twin Peaks" –
modest-size hills – about a kilometer (five-eighths of a mile)
away

Mars also has permanent ice caps at both poles, which retreat and grow with
the seasons. For more about them, see Mars, polar
caps.

Geology

Although tectonism has clearly played
a major role in the planet's development, it has not involved the lateral
movement of sliding plates as on our own world, but instead only vertical
movement of hot lava pushing up to the surface. The lack of plate
tectonics, as on Mercury and the Moon, has
resulted in hot-spots remaining fixed in certain locations beneath the crust.
This, along with the lower surface gravity, may account for the Tharsis bulge
and its enormous volcanoes. Although there's no evidence of current volcanic
activity, observations by Mars Global Surveyor suggest that Mars may have
been tectonically active early on, making comparisons with Earth all the
more interesting. Lacking plate tectonics today, Mars can't recycle any
of the carbon dioxide in its rocks back into its atmosphere and so can't
sustain much of a greenhouse effect. Large, but not global, weak magnetic
fields exist in various regions of Mars, probably remnants of an earlier
global field that has since disappeared.

In many places, there is incontrovertible evidence of erosion and a watery
past, including flood plains and river channels, which tantalize the astrobiologist
(see Mars, water). Valles Marineris, however,
was created not by running water but by the stretching and cracking of the
crust associated with the formation of the Tharsis bulge.

Interior

Inside, Mars may have a core roughly 1,700km in radius which the planet's
relatively low density suggests contains a higher ratio of sulfur to iron
than that in the cores of the other terrestrial worlds. Overlaying this
is probably a molten, rocky mantle, somewhat denser than the Earth's, topped
with a thin crust, which, based on data collected by Mars Global Surveyor,
is about 80km thick in the southern hemisphere and about 35 km thick in
the north.

Atmosphere

Mars has a thin atmosphere, which is composed of mainly (95.3%) carbon dioxide.
Other gases present include nitrogen (2.7%), argon (1.6%), oxygen (0.15%),
water vapor (0.03%), and, intriguingly, methane. The discovery of a bit
of methane in the martian atmosphere has excited scientists because they
don't know where it has come from. One possibility is geological activity
below the surface, another is microbes giving off the gas as part of their
metabolism. For more, see Mars, atmosphere.

Life and water on Mars

People have long wondered if there might be life on the fourth planet. For
more about this whole subject, see Mars, life.
In the second half of the 19th century, speculation about Martians stepped
up a gear when some observers reported seeing channels, or what came to
be called "canals," on the surface. The seasonal waxing and waning of the
martian polar caps was seen by early observers to be accompanied by surface
changes at lower latitudes (see Mars, changes).
The darkening of certain regions, following each spring thaw, prompted theories
of inundation by floodwater and the growth of vast tracts of plant life.
See the encyclopedia entries on the canals of
Mars and Mars, vegetation.

Ever since then, questions about the existence of life and water on Mars
have been entwined. Observations by spacecraft have shown that, in the remote
past, conditions on the fourth planet were friendlier to life as we know
it than they are today. Mars has been through periods when it was warmer
and wetter than it is now (see Mars, past conditions).
However, we still don't know if life managed to develop under those milder
conditions. And, if it did, whether it survived to the present day.